The process of removing heat from an aqueous solution is known as "cooling". One method of removing this heat is by the application of heat exchangers and forced evaporative structures, known as cooling towers, in series with one another. The transport vehicle is generally water. As this water is recirculated, it concentrates due to the evaporative nature of the process and consequently, produces undesirous side effects in the associated pipes, pumps, and heat exchangers. These deleterious side effects are corrosion, scaling and fouling. To combat this effect, chemicals are added to the system.
Three of the most widely added chemicals as corrosion inhibitors are chromium in its chromate and dichromate ionic forms, zinc in its ionic form, and phosphates, the chromates and zinc being toxic in high concentrations. While each of these added alone, act as a corrosion inhibitor, each must be added in high concentrations to be effective. In particular, zinc in its ionic form, should be added in concentrations of between about 1 ppm. and about 5 ppm. However, because to date, both selective removal of zinc and effective treatment of the effluent to meet environmental regulations in relation to the zinc discharged into the natural waterways, only about 1 ppm. of zinc in its ionic form is added to the cooling water.
When these chemicals are mixed however they synergistically enhance each other and enable lower concentrations to be used. This fact is important from both economic and environmental considerations. Of the three possible combinations of these chemicals, the most effective is believed to be the combination of zinc and chromium in its chromate and dichromate forms, rapid establishment of protection being characteristic of the zinc, but durability being provided by the chromate. When mixed, the chromates and zinc synergistically enhance each other, the combination being most effective at a ratio of [Cr.sup.6+ ] in the chromates:[Zn.sup.2+ ] of between about 3:1 and about 4:1 by weight. For good corrosion control, between about 12 ppm. and about 15 ppm. of Cr.sup.6+ is preferred. Consequently, at least about 3 ppm of Zn.sup.2+ is required for good corrosion inhibition. Once again, environmental regulations preclude their effective use.
If therefore, greater concentrations of zinc and chromium are to be employed, and together, to provide effective corrosion inhibition, these materials must be recovered--from a cost point of view, these materials are lost if discharged and from an environmental point of view, they would cause irreparable damage to the ecological balance.
Three possible routes exist for their removal.
1. chemical reduction or precipitation;
2. electrochemical reduction and precipitation;
3. ion exchange.
Of the three, ion exchange is the only method where the zinc and chromium are recovered in a form directly reusable in the system from which they were removed. It is also the area to which this invention relates.
Most anion exchange resins exhibit excellent selectivities for chromate (CrO.sub.4).sup.2- and dichromate (Cr.sub.2 O.sub.7).sup.2- anionic species. Both weakly basic and strongly basic anion ion exchange resins can be used for selective chromate removal, but the strongly basic resins are prone to fouling by silica and organics. Also, in order to remove the chromates and dichromates from the strongly basic resin, regeneration with a caustic/brine solution is necessary. The weakly basic resins on the other hand are more resistant to fouling and can be regenerated using caustic alone. For these reasons, the recent trend has been to use weakly basic resins. One such resin is Amberlite IRA-94.TM. manufactured by Rohm and Haas. After a service run, IRA-94.TM. is regenerated with sodium hydroxide to remove and recover the chromium, leaving the resin in its free base form. In order to convert it back to the anionic form, it must be conditioned with a suitable acid.
The overall service and regeneration cycle is summarized as follows: